1. Field of the Invention
This invention relates to a multi-countershaft transmission and, more particularly, to a multi-countershaft or twin countershaft transmission with a floating main shaft.
2. Description of the Prior Art
The advantages of multi-countershaft transmissions are well known as well as the disadvantages associated with them.
In multi-speed automotive transmissions carrying heavy torque loads, the single countershaft transmission becomes quite large. A multi-countershaft transmission is attractive to reduce size, carry heavier torque loads, increase gear life, and reduce costs. The primary problem associated with the use of multi-countershafts is that of insuring that the torque load is evenly divided among the countershafts. Obviously, the above purposes are thwarted if there isn't an equal division of torque. Precision machining of the transmission components, particularly the gears, shafts, and housing to guarantee equal load distribution can become cost prohibitive.
Consideration of the foregoing problems have lead to the popularity of a twin countershaft transmission. When there are only two countershafts, adjustment of the main shaft between the two countershafts will balance the torque. Thus, a floating main shaft has been touted as the answer to the problem; because, if the main shaft is truly floating, it will seek its own balance between the two countershafts. While some prior art multiple or twin countershaft transmissions have been labelled as having a floating main shaft, usually they have permitted a limited amount of movement at the input or head end of the main shaft with a decreasing mobility toward the outlet end which at best might be pivoted.
While external bearings at the input end of the main shaft have in some instances been replaced by bushings with large clearances, or by internal bearings or piloting on the input shaft, an external roller bearing has usually been retained at the output end of the main shaft.
Another early technique was to mount the main shaft in a forward and a rearward (input and output end) roller bearing held in spring loaded cradles which allows limited movement of the shaft at each end.
Commonly, the output gear of the main shaft, most commonly referred to as the range drive gear, is mounted on a bearing in the casing with the main shaft fitting inside this output gear allowing no movement. Even where the connection between the output gear and shaft is with a loosely fitting spline, for all practical purposes, the shaft is fixed because as soon as torque is applied on the spline, it sticks in a single position and does not move around or become self adjusting. With the output end of the shaft fixed in a given position because it cannot slide under torque, there cannot be a true equal torque split.